netdev - Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh

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Date: Sun, 21 May 2023 20:22:16 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Alexander Aring <alex.aring@...il.com>, 
	David Lebrun <david.lebrun@...ouvain.be>, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net 1/3] ipv6: exthdrs: fix potential use-after-free in ipv6_rpl_srh_rcv()

On Thu, May 18, 2023 at 7:05 PM Simon Horman <simon.horman@...igine.com> wrote:
>
> On Wed, May 17, 2023 at 09:31:16PM +0000, Eric Dumazet wrote:
> > After calls to pskb_may_pull() we need to reload @hdr variable,
> > because skb->head might have changed.
> >
> > We need to move up first pskb_may_pull() call right after
> > looped_back label.
> >
> > Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Alexander Aring <alex.aring@...il.com>
> > Cc: David Lebrun <david.lebrun@...ouvain.be>
> > ---
> >  net/ipv6/exthdrs.c | 11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> > index a8d961d3a477f6516f542025dfbcfc6f47407a70..b129e982205ee43cbf74f4900c3031827d962dc2 100644
> > --- a/net/ipv6/exthdrs.c
> > +++ b/net/ipv6/exthdrs.c
> > @@ -511,6 +511,10 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
> >       }
> >
> >  looped_back:
> > +     if (!pskb_may_pull(skb, sizeof(*hdr))) {
> > +             kfree_skb(skb);
> > +             return -1;
> > +     }
> >       hdr = (struct ipv6_rpl_sr_hdr *)skb_transport_header(skb);
> >
> >       if (hdr->segments_left == 0) {
>
> Hi Eric,
>
> Not far below this line there is a call to pskb_pull():
>
>                 if (hdr->nexthdr == NEXTHDR_IPV6) {
>                         int offset = (hdr->hdrlen + 1) << 3;
>
>                         skb_postpull_rcsum(skb, skb_network_header(skb),
>                                            skb_network_header_len(skb));
>
>                         if (!pskb_pull(skb, offset)) {
>                                 kfree_skb(skb);
>                                 return -1;
>                         }
>                         skb_postpull_rcsum(skb, skb_transport_header(skb),
>                                            offset);
>
> Should hdr be reloaded after the call to pskb_pull() too?

I do not think so, because @hdr is not used between this pskb_pull()
and the return -1:

       if (hdr->nexthdr == NEXTHDR_IPV6) {
            int offset = (hdr->hdrlen + 1) << 3;

            skb_postpull_rcsum(skb, skb_network_header(skb),
                       skb_network_header_len(skb));

            if (!pskb_pull(skb, offset)) {
                kfree_skb(skb);
                return -1;
            }
            skb_postpull_rcsum(skb, skb_transport_header(skb),
                       offset);

            skb_reset_network_header(skb);
            skb_reset_transport_header(skb);
            skb->encapsulation = 0;

            __skb_tunnel_rx(skb, skb->dev, net);

            netif_rx(skb);
            return -1;
        }